Safety mechanisms, wake up and shutdown methods in distributed power installations

ABSTRACT

A distributed power system including multiple DC power sources and multiple power modules. The power modules include inputs coupled respectively to the DC power sources and outputs coupled in series to form a serial string. An inverter is coupled to the serial string. The inverter converts power input from the serial string to output power. A signaling mechanism between the inverter and the power module is adapted for controlling operation of the power modules.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No. 12/329,525 filed Dec. 5, 2008, which is a continuation-in-part of U.S. application Ser. No. 11/950,271 filed Dec. 4, 2007 and from U.S. Provisional Application Ser. No. 60/992,589 filed Dec. 5, 2007, the disclosures of which are included herein by reference for all purposes as if entirely set forth herein.

FIELD AND BACKGROUND

The present invention relates to distributed power systems and, more particularly, wake-up and shutdown algorithms for the photovoltaic distributed power systems.

Utility networks provide an electrical power system to utility customers. The distribution of electric power from utility companies to customers utilizes a network of utility lines connected in a grid-like fashion, referred to as an electrical grid. The electrical grid may consist of many independent energy sources energizing the grid in addition to utility companies energizing the grid, with each independent energy source being referred to as a distributed power (DP) generation system. The modern utility network includes the utility power source, consumer loads, and the distributed power generation systems which also supply electrical power to the network. The number and types of distributed power generation systems is growing rapidly and can include photovoltaics, wind, hydro, fuel cells, storage systems such as battery, super-conducting flywheel, and capacitor types, and mechanical devices including conventional and variable speed diesel engines, Stirling engines, gas turbines, and micro-turbines. These distributed power generation systems are connected to the utility network such that they operate in parallel with the utility power sources.

A conventional installation of a solar distributed power system 10, including multiple solar panels 101, is illustrated in FIG. 1. Since the voltage provided by each individual solar panel 101 is low, several panels 101 are connected in series to form a string 103 of panels 101. For a large installation, when higher current is required, several strings 103 may be connected in parallel to form overall system 10. The interconnected solar panels 101 are mounted outdoors, and connected to a maximum power point tracking (MPPT) module 107 and then to an inverter 104. MPPT 107 is typically implemented as part of inverter 104 as shown in FIG. 1. The harvested power from DC sources 101 is delivered to inverter 104, which converts the direct-current (DC) into alternating-current (AC) having a desired voltage and frequency, which is usually 110V or 220V at 60 Hz, or 220V at 50 Hz. The AC current from inverter 104 may then be used for operating electric appliances or fed to the power grid.

As noted above, each solar panel 101 supplies relatively very low voltage and current. A problem facing the solar array designer is to produce a standard AC current at 120V or 220V root-mean-square (RMS) from a combination of the low voltages of the solar panels. The delivery of high power from a low voltage requires very high currents, which cause large conduction losses on the order of the second power of the current i². Furthermore, a power inverter, such as inverter 104, which is used to convert DC current to AC current, is most efficient when its input voltage is slightly higher than its output RMS voltage multiplied by the square root of 2 (which is the peak voltage). Hence, in many applications, the power sources, such as solar panels 101, are combined in order to reach the correct voltage or current. A large number of panels 101 are connected into a string 103 and strings 103 are connected in parallel to power inverter 104. Panels 101 are connected in series in order to reach the minimal voltage required for inverter 104. Multiple strings 103 are connected in parallel into an array to supply higher current, so as to enable higher power output.

FIG. 1B illustrates one serial string 103 of DC sources, e.g., solar panels 101 a-101 d, connected to MPPT circuit 107 and inverter 104. The current versus voltage (IV) characteristics is plotted (110 a-110 d) to the left of each DC source 101. For each DC power source 101, the current decreases as the output voltage increases. At some voltage value, the current goes to zero, and in some applications the voltage value may assume a negative value, meaning that the source becomes a sink. Bypass diodes (not shown) are used to prevent the source from becoming a sink. The power output of each source 101, which is equal to the product of current and voltage (P=i*V), varies depending on the voltage drawn from the source. At a certain current and voltage, close to the falling off point of the current, the power reaches its maximum. It is desirable to operate a power generating cell at this maximum power point (MPP). The purpose of the MPPT is to find this point and operate the system at this point so as to draw the maximum power from the sources.

In a typical, conventional solar panel array, different algorithms and techniques are used to optimize the integrated power output of system 10 using MPPT module 107. MPPT module 107 receives the current extracted from all of solar panels 101 together and tracks the maximum power point for this current to provide the maximum average power such that if more current is extracted, the average voltage from the panels starts to drop, thus lowering the harvested power. MPPT module 107 maintains a current that yields the maximum average power from system 10.

However, since power sources 101 a-101 d are connected in series to single MPPT 107, MPPT 107 selects a maximum power point which is some average of the maximum power points of the individual serially connected sources 101. In practice, it is very likely that MPPT 107 would operate at an I-V point that is optimum for only a few or none of sources 101. In the example of FIG. 1B, the selected point is the maximum power point for source 101 b, but is off the maximum power point for sources 101 a, 101 c and 101 d. Consequently, the arrangement is not operated at best achievable efficiency.

The present applicant has disclosed in co-pending U.S. application Ser. No. 11/950,271 entitled “Distributed Power Harvesting Systems Using DC Power Sources”, the use of an electrical power converter, e.g. DC-to-DC converter, attached to the output of each power source, e.g. photovoltaic panel. The electrical power converter converts input power to output power by monitoring and controlling the input power at a maximum power level.

The term “signaling” or “signaling mechanism” as used herein refers to either a signal modulated on an electromagnetic carrier signal or a simple unmodulated signal such as an on/off signal “keep alive” signal or “dry contact” signal. For a modulated signal, the modulation method may be by any such method known in the art by way of example, frequency modulation (FM) transmission, amplitude modulation (AM), FSK (frequency shift keying) modulation, PSK (phase shift keying) modulation, various QAM (Quadrature amplitude modulation) constellations, or any other method of modulation.

The term “power module” as used herein includes power converters such as a DC-DC power converter but also includes modules adapted to control the power passing through the module or a portion of the power, whether by switching or other means.

SUMMARY

The following summary of the invention is included in order to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention and as such it is not intended to particularly identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below.

According to an aspect of the present invention, there is provided a distributed power system including a DC power source and a power module. The power module includes an input coupled respectively to the DC power source and an output. An inverter is coupled to the output. The inverter converts power input from the output of the power module to output power. A signaling mechanism between the inverter and the power module is adapted for controlling operation of the power module. During operation of the distributed power system, in some embodiments, the signaling mechanism may superimpose a signal on the output of the power module. The signaling mechanism may include a switch integrated with the inverter, the switch modulating the signal onto the output of the power module. A receiver integrated with the power modules receives the signal from the inverter. Alternatively a detection mechanism in the power module detects a signal at the frequency of the electrical grid. Alternatively, a signal from the electrical grid is detected in the output of the power module at a higher frequency up-converted from the frequency of the electrical grid. Alternatively, a detection mechanism in the power module detects a switching frequency of the inverter. The power modules are may be configured for operation in a safety mode, and during the safety mode, the power at the output of the power module, the voltage across the output of the power module, and/or the current flowing through it, are limited so as not to endanger personnel. The power module may include a detection mechanism wherein during operation of the distributed power system, the detection mechanism detects a signal from the inverter. Based on the signal, the operation of the power module is varied from the safety mode of operation to a normal mode of operation for converting power of the DC power source from the input to the output of the power module.

According to another aspect of the present invention there is provided a method for operating a distributed power system. The system includes a DC power source and a power module. The power module includes an input coupled to the DC power source. The power module includes an output. An inverter is coupled to the output of the power module. The inverter converts a power output from the power module to an output power. The method includes operating the power modules in a safety mode by limiting the power output from the power module. The safety mode is characterized by having less than a predetermined amount (e.g. ten milliamperes) of current flow and/or less than a predetermined amount (e.g. 2 Volts) through the output of the power module. A signal from the inverter is preferably monitored and upon detecting the signal from the inverter, the power input to the inverter is increased by operating the power module in a normal mode of operation for converting power of the DC power source from the input to the output of the power module. Upon detecting the signal and prior to the operation of the power module in the normal mode of operation, the voltage of the output of the power module is preferably ramped up slowly. The normal mode of operation of the power module may include controlling a maximum peak power at the input coupled to the DC power sources.

The foregoing and/or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, exemplify embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate various features of the illustrated embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not necessarily drawn to scale.

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1 is a block diagram of a conventional power harvesting system using photovoltaic panels as DC power sources;

FIG. 1B illustrates current versus voltage characteristic curves for one serial string the DC power sources of FIG. 1;

FIG. 2 is a simplified block diagram illustrating a distributed power harvesting circuit, based on the disclosure in U.S. application Ser. No. 11/950,271, according to an aspect of the present invention;

FIG. 2A is a simplified block diagram of a DC-to-DC converter, including a feature of the present invention;

FIG. 3 illustrates an exemplary DC-to-DC converter, is a simplified block diagram illustrating in more detail;

FIG. 4 is a simplified block diagram of another exemplary system, according to an embodiment of the present invention;

FIG. 4A is a simplified block diagram illustrating in more detail, a power module according to the embodiment of FIG. 4;

FIG. 4B is a simplified block diagram illustrating in more detail, a signaling mechanism attached to a conventional inverter, according to embodiments of the present invention;

FIG. 5 is a simplified flow diagram illustrating a method for wake-up and shutdown of a power harvesting system with a safety mode, according to a feature of the present invention;

FIG. 5A is a flow diagram illustrating methods for wake-up and shutdown of a power harvesting system, according to embodiments of the present invention, the flow diagram including method steps performed by the power converters/modules; and

FIG. 6 is another flow diagram illustrating methods for wake-up and shutdown of a power harvesting system, according to embodiments of the present invention, the flow diagram including method steps performed by the inverter of FIG. 2 or signaling block of FIG. 4B.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures.

It should be noted, that although the discussion herein relates primarily to wake-up and shutdown methods in photovoltaic systems and more particularly to those systems previously disclosed in U.S. application Ser. No. 11/950,271, the present invention may, by non-limiting example, alternatively be configured as well using conventional photovoltaic distributed power systems and other distributed power systems including (but not limited to) wind turbines, hydroturbines, fuel cells, storage systems such as battery, super-conducting flywheel, and capacitors, and mechanical devices including conventional and variable speed diesel engines, Stirling engines, gas turbines, and micro-turbines.

By way of introduction, it is important to note that aspects of the present invention have important safety benefits. While installing or performing maintenance on photovoltaic systems according to certain aspects of the present invention, installers are protected from danger of shock or electrocution since systems according to embodiments of the present invention do not output potentially dangerous high voltage and/or currents when an operational inverter is not connected during installation and maintenance procedures.

Before explaining embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details of design and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

Reference is now made to FIG. 2 which illustrates a distributed power harvesting circuit 20, based on the disclosure in U.S. application Ser. No. 11/950,271. Circuit 20 enables connection of multiple distributed power sources, for example solar panels 101 a-101 d, to a single power supply. Series string 203 of solar panels 101 may be coupled to an inverter 204 or multiple connected strings 203 of solar panels 101 may be connected to a single inverter 204. In configuration 20, each solar panel 101 a-101 d is connected individually to a separate power converter circuit or a module 205 a-205 d. Each solar panel 101 together with its associated power converter circuit 205 forms a power generating element 222. (Only one such power generating element 222 is marked in FIG. 2.) Each converter 205 a-205 d adapts optimally to the power characteristics of the connected solar panel 101 a-101 d and transfers the power efficiently from input to output of converter 205. Converters 205 a-205 d are typically microprocessor controlled switching converters, e.g. buck converters, boost converters, buck/boost converters, flyback or forward converters, etc. The converters 205 a-205 d may also contain a number of component converters, for example a serial connection of a buck and a boost converter. Each converter 205 a-205 d includes a control loop 221, e.g. MPPT loop that receives a feedback signal, not from the converter's output current or voltage, but rather from the converter's input coming from solar panel 101. The MPPT loop of converter 205 locks the input voltage and current from each solar panel 101 a-101 d at its optimal power point, by varying one or more duty cycles of the switching conversion typically by pulse width modulation (PWM) in such a way that maximum power is extracted from each attached panel 101 a-101 d. The controller of converter 205 dynamically tracks the maximum power point at the converter input. Feedback loop 221 is closed on the input power in order to track maximum input power rather than closing a feedback loop on the output voltage as performed by conventional DC-to-DC voltage converters.

As a result of having a separate MPPT circuit in each converter 205 a-205 d, and consequently for each solar panel 101 a-101 d, each string 203 may have a different number or different specification, size and/or model of panels 101 a-101 d connected in series. System 20 of FIG. 2 continuously performs MPPT on the output of each solar panel 101 a-101 d to react to changes in temperature, solar radiance, shading or other performance factors that effect one or more of solar panels 101 a-101 d. As a result, the MPPT circuit within the converters 205 a-205 d harvests the maximum possible power from each panel 101 a-101 d and transfers this power as output regardless of the parameters effecting other solar panels 101 a-101 d.

The outputs of converters 205 a-205 d are series connected into a single DC output that forms the input to inverter 204. Inverter 204 converts the series connected DC output of converters 205 a-205 d into an AC power supply. Inverter 204, regulates the voltage at the input of inverter 204. In this example, an independent control loop 220 holds the voltage input to inverter 204 at a set value, say 400 volts. The current at the input of inverter 204 is typically fixed by the power available and generated by photovoltaic panels 101.

According to a feature of the present invention, information regarding wakeup or shut-down may be conveyed from inverter 204 to converters 205. The information may be transmitted using any of the methods well known to those experienced in the art. According to certain embodiments, a modulation method may be used, by way of example, frequency modulation (FM) transmission, amplitude modulation (AM), FSK (frequency shift keying) modulation, PSK (phase shift keying) modulation, various QAM (Quadrature amplitude modulation) constellations, or any other method of modulation. Alternatively, inverter 204, while converting power from its input to its output, actively creates a frequency ripple in serial string 203. During normal operation, the 100 Hz (or 120 Hz in USA) ripple is detectable in serial string 203 since the capacitors of inverter 204 do not entirely block the alternating current (AC), and an additional signaling mechanism is not required to produce the 100/120 Hz signal in serial string 203. Alternatively or in addition, one or more switching frequencies of inverter 204, typically 16 Khz or 32 KHz may be detectable as leakage or provided intentionally to serial string 203.

Reference is now made to FIG. 2A which illustrates a feature of the present invention. In FIG. 2A, converter 205 is shown in more detail. Integrated with power converter 205 is a detector/receiver 207, according to a feature of the present invention which is configured to receive, optionally amplify and detect the signal, e.g at 100/120 Hz originating in inverter 204.

Controller 306 preferably either polls a signal input 209 from receiver/detector 207 or uses signal input 209 as an interrupt so that only when detector/receiver 207 detects the 100/120 Hz signal, is module 205 in a normal operating mode converting power from its input to its output. Receiver 207 is alternatively configured to detect the 16/32 KHz inverter switching frequency and provides an enabling signal to controller on signal input 209 while inverter 204 is operating.

Reference is now made to FIG. 3 which illustrates an exemplary DC-to-DC converter 205, according to a feature of the present invention. DC-to-DC converters are used to either step down or step up a DC voltage input to a higher or a lower DC voltage output, depending on the requirements of the output circuit. However, in the embodiment of FIG. 3 the DC-DC converter 205 is used as a power converter, i.e., transferring the input power to output power, the input voltage varying according to the MPPT at the input, while the output current is dictated by the constant input voltage to inverter 104, 204. That is, the input voltage and current may vary at any time and the output voltage and current may vary at any time, depending on the operating condition of DC power sources 101.

Converter 205 is connected to a corresponding DC power source 101 at input terminals 314 and 316. The converted power of the DC power source 101 is output to the circuit through output terminals 310, 312. Between the input terminals 314, 316 and the output terminals 310, 312, the converter circuit includes input and output capacitors 320, 340, backflow prevention diodes 322, 342 and a power conversion circuit including a controller 306 and an inductor 308.

Diode 342 is in series with output 312 with a polarity such that current does not backflow into the converter 205. Diode 322 is coupled between the positive output lead 312 through inductor 308 which acts a short for DC current and the negative input lead 314 with such polarity to prevent a current from the output 312 to backflow into solar panel 101.

A potential difference exists between wires 314 and 316 due to the electron-hole pairs produced in the solar cells of panel 101. Converter 205 maintains maximum power output by extracting current from the solar panel 101 at its peak power point by continuously monitoring the current and voltage provided by panel 101 and using a maximum power point tracking algorithm. Controller 306 includes an MPPT circuit or algorithm for performing the peak power tracking Peak power tracking and pulse width modulation (PWM) are performed together to achieve the desired input voltage and current. The MPPT in controller 306 may be any conventional MPPT, such as, e.g., perturb and observe (P&O), incremental conductance, etc. However, notably the MPPT is performed on panel 101 directly, i.e., at the input to converter 205, rather than at the output of converter 205. The generated power is then transferred to the output terminals 310 and 312. The outputs of multiple converters 205 may be connected in series, such that the positive lead 312 of one converter 205 is connected to the negative lead 310 of the next converter 205.

In FIG. 3, converter 205 is shown as a buck plus boost converter. The term “buck plus boost” as used herein is a buck converter directly followed by a boost converter as shown in FIG. 3, which may also appear in the literature as “cascaded buck-boost converter”. If the voltage is to be lowered, the boost portion is substantially shorted. If the voltage is to be raised, the buck portion is substantially shorted. The term “buck plus boost” differs from buck/boost topology which is a classic topology that may be used when voltage is to be raised or lowered, and sometimes appears in the literature as “cascaded buck-boost”. The efficiency of “buck/boost” topology is inherently lower then a buck or a boost. Additionally, for given requirements, a buck-boost converter will need bigger passive components then a buck plus boost converter in order to function. Therefore, the buck plus boost topology of FIG. 3 has a higher efficiency than the buck/boost topology. However, the circuit of FIG. 3 continuously decides whether it is bucking or boosting. In some situations when the desired output voltage is similar to the input voltage, then both the buck and boost portions may be operational.

The controller 306 may include a pulse width modulator, PWM, or a digital pulse width modulator, DPWM, to be used with the buck and boost converter circuits. Controller 306 controls both the buck converter and the boost converter and determines whether a buck or a boost operation is to be performed. In some circumstances both the buck and boost portions may operate together. That is, the input voltage and current are selected independently of the selection of output current and voltage. Moreover, the selection of either input or output values may change at any given moment depending on the operation of the DC power sources. Therefore, in the embodiment of FIG. 3, converter 205 is constructed so that at any given time a selected value of input voltage and current may be up converted or down converted depending on the output requirement.

In one implementation, an integrated circuit (IC) 304 may be used that incorporates some of the functionality of converter 205. IC 304 is optionally a single ASIC able to withstand harsh temperature extremes present in outdoor solar installations. ASIC 304 may be designed for a high mean time between failures (MTBF) of more than 25 years. However, a discrete solution using multiple integrated circuits may also be used in a similar manner. In the exemplary embodiment shown in FIG. 3, the buck plus boost portion of the converter 305 is implemented as the IC 304. Practical considerations may lead to other segmentations of the system. For example, in one aspect of the invention, the IC 304 may include two ICs, one analog IC which handles the high currents and voltages in the system, and one simple low-voltage digital IC which includes the control logic. The analog IC may be implemented using power FETs which may alternatively be implemented in discrete components, FET drivers, A/Ds, and the like. The digital IC may form controller 306.

In the exemplary circuit 205 shown, the buck converter includes input capacitor 320, transistors 328 and 330, diode 322 positioned in parallel to transistor 328, and inductor 308. Transistors 328, 330 each have a parasitic body diode 324, 326. The boost converter includes inductor 308, which is shared with the buck converter, transistors 348 and 350 a diode 342 positioned in parallel to transistor 350, and output capacitor 340. Transistors 348, 350 each have a parasitic body diode 344, 346.

System 20 includes converters 205 which are connected in series and carry the current from string 203. If a failure in one of the serially connected converters 205 causes an open circuit in failed converter 205, current ceases to flow through the entire string 203 of converters 205, thereby causing system 20 to stop functioning. Aspects of the present invention provide a converter circuit 205 in which electrical components have one or more bypass routes associated with them that carry the current in case of an electrical component failing within one of converters 205. For example, each switching transistor of either the buck or the boost portion of the converter has its own diode bypass. Also, upon failure of inductor 308, the current bypasses the failed inductor 308 through parasitic diodes 344,346.

In FIG. 3, detector/receiver block 207 is shown which is configured to provide an enable signal 209 to microcontroller 306 when the communications signal originating in inverter 104,204 is detected.

Reference in now made to FIG. 4, which illustrate system 40, according to an embodiment of the present invention. For simplicity, a single string 423 is shown of distributed power sources, e.g. solar panels 101 a-101 d connected to respective power modules 405 a-d. Serial string 423 is input to conventional inverter 104 through wires 412 and 410. The output of inverter 104 is connected to and supplies electrical power to the electrical grid. At the input of inverter 104, is connected a signaling mechanism 420 which superimposes a signal on serial string 423 through wires 412 and 410 when inverter 104 is converting power to the grid.

Reference is now also made to FIG. 4B which illustrates in more detail signaling mechanism 420. Signaling mechanism 420 includes a relay 428 which is normally open and controlled by a microcontroller 422. Relay 428 is switched at a given rate, e.g. 100 Hz, and the signal is superimposed by action of relay 428 onto serial string 423 over wires 410 and 412. Microcontroller 422 typically provides the control of the signal, e.g. 100 Hz, during normal operation of distributed power system 40. Microcontroller 422 is typically connected to one or more sensors in order to monitor the operation of inverter 104. In the example of FIG. 4B, microcontroller 422 monitors over-voltage of the input DC voltage to inverter 104. The example shown in FIG. 4B includes an input DC voltage tap 432 connected to an analog to digital converter (A/D) 430, the output of which is provided to microcontroller 422. The tap 432 may be, e.g., a Hall-effect sensors, series connected resistor across which the voltage drop is measured, etc. In one embodiment, an over-voltage condition as measured by microcontroller 422, results in microcontroller 422 stopping the signaling through relay 428 and/or opening one or more protective relays 424, 426 in series with the input DC voltage to inverter 104. Note that one switch 424 or 426 may be enough for performing the required action, and two switches in series are shown solely for the purpose of illustration that double protection might be required by some regulatory bodies. A power management block 434 taps voltage for powering microcontroller 422 and any other active electronics components (not shown) in block 420.

Reference is now made to FIG. 4A which illustrates in more detail certain aspects of power module 405. Integrated with power module 405 is detector/receiver 207 which is configured to receive, optionally amplify and detect the signal, e.g. at 100 Hz, produced by signal mechanism 420. Controller 306 preferably either polls signal input 209 or uses signal input 209 as an interrupt so that only when detector/receiver 207 detects the 100 Hz signal, is module 405 operating in a normal operating mode. Power module 405 is shown to include a bypass diode 414. Optionally, power module 405 may include a conventional DC/DC switching converter with a control loop based on output power. Power module 405 includes at least one switch 416 controlled by controller 306 which functions to stop normal operation of power from the input of module 405 to the output of 405 when signal input 209 is absent indicating that inverter 104 is not transferring power to the electrical grid.

Reference is now made to FIG. 5 which illustrates a simplified method for safe operation of system 40, according to an aspect of the present invention. In step 501, active control circuits, e.g. microcontroller 306, are turned on. Module 205, 405 begins operation (step 53) in a safety mode. In safety mode, output current and/or voltage from module 405 is limited, for instance output voltage is limited to 2 volts and output current is limited to 10 mA so that a person can touch the wires of serial string 203, 423 without any danger of electrocution.

Controller 306 maintains safety mode operation (step 53) until a communications signal, e.g. 100 Hz, is received (decision box 505) by receiver/detector 207 from inverter 204 or signaling block 420. When the communications signal is received (decision block 505) indicating inverter 104 or 204 is connected and converting power, safety mode (step 53) of operation ends. When the communications signal is received (decision block 505), module 405 preferably enters a normal operation mode (step 57), typically with maximum power point tracking. The normal operation of transferring power is maintained as long as the communications signal, e.g. 100 Hz is received from inverter 204 or signal mechanism 420, and no other warning condition is present. If the communications signal is not detected, or another warning condition is present, the normal mode (step 57) is typically ended and power conversion of modules 405 is typically turned off. If in decision box 509, the communications signal is not detected, or another warning condition is present, the normal mode (step 57) is typically ended and power conversion of modules 405 is typically turned off.

Reference is now made to FIG. 5A, which illustrates a method 50 for wake-up and shutdown of module 405, according to embodiments of the present invention. Method 50 is applicable to both systems 20 and 40. In step 501, active control circuits, e.g. microcontroller 306, are turned on. Active control circuits are typically turned on (step 501) in the early morning when there is sufficient light to power the active control circuits typically with voltage of DC voltage source 101 reaching three volts. In decision block 503, when voltage output—or power output—from DC voltage source 101 is sufficiently high and stable (e.g. voltage input to module 405 is ten volts for a period of 30 seconds), then module 205,405 begins operation (step 53) in a safety mode. In safety mode, output current and/or voltage from module 405 is limited, for instance output voltage is limited to 2 volts and output current is limited to 10 mA so that a person can touch the wires of serial string 203,423 without any danger of electrocution. Note also, that in this case even if 25 modules are connected in series, the maximum output voltage of the string doesn't exceed 50V—which means the string voltage is still safe. Referring back to FIG. 3, safety mode may be achieved by controller 306 in module 405 by turning on FET 330 and turning off FETS 328, 348, and 350. Output wire 412 is held close to zero volts. Alternatively, the controller 306 may alternate the switches (e.g. switches 324 & 326 of buck converter) at a low duty-cycle in order to maintain a low output voltage.

Referring back to FIG. 5A, controller 306 maintains safety mode operation (step 53) until a communications signal, e.g. 100 Hz, is received by receiver/detector 207 from inverter 204 or signaling block 420. When the communications signal is received (decision block 505) indicating inverter 104 or 204 is connected and converting power, safety mode (step 53) of operation ends. When the communications signal is received (decision block 505), module 405 preferably enters a voltage control mode (step 55) and voltage output between wires 412,410 is slowly ramped up. Voltage continues to ramp up, typically as high as +60V until module 205,405 detects that current is being drawn (step 507). When sufficient current is drawn (step 507), module 205, 405 begins normal operation, (step 57) e.g. for module 205, the normal mode is the maximum power point (MPP) tracking mode of converting DC power from its input to its output by maintain maximum power at its input. The normal operation of transferring power is maintained as long as the communications signal, e.g. 100 Hz is received from inverter 204 or signal mechanism 420, and no other warning condition is present. If the communications signal is not detected, or another warning condition is present, the normal mode (step 57) is typically ended and power conversion of modules 405 is typically turned off. Exemplary warning conditions in decision box 509, which cause module 205,405 to end normal mode (step 57) and to stop transferring power to its output include: (i) input voltage less than predetermined value, e.g. about 10 volts for 5 seconds, (ii) rapid change in output voltage, for instance greater than 20% in 100 milliseconds, (iii) reception of signal requesting to stop producing power, (iv) not receiving a signal to produce power (in the case where recurring “allow production” signals are required for the converter to function), or (v) output exceeds over voltage threshold caused for instance when multiple modules 205 in string 203 are converting power (step 57) and one of modules 205 of string 203 shuts down, then the other modules 205 of string 203 have a raise of output voltage.

Reference is now made to FIG. 6, which illustrates a method 60 performed by inverter 204 or signaling block 420 attached at the input of inverter 104. In step 601, inverter 104 is off or inverter 204 is on standby, and not converting power to its output. In decision box 603, start conditions for turning on inverter 104,204 are determined. Typically, as a safety requirement, inverter 104 delays operation (converting power to its output) until after at least 5 minutes of connection to a functioning AC-grid at its output. This safety requirement may be achieved using microcontroller 422 and at least one of relays 424 and 426 in signaling block 420. In inverter 204, a minimum voltage is required at the input to inverter 204 (e.g. if the safety output voltage of each module is 2V, and the minimal-length string allowed contains 5 modules, the inverter will wait until at least 10V are present at its DC input) and only thereafter does inverter 204 begin to charge its input, typically to a specified standard input of 400V.

In step 605, communications signal, e.g 100 Hz, is superimposed on serial string 203,423 either from signaling mechanism 420 or from inverter 204 for instance when at least a 50 Watt load is attached to the output of inverter 204. In decision box 607, when the specified input voltage is reached, e.g 400V for inverter 204, inverter 204 is turned on or inverter 104 is attached to serial string 423 by mechanism 420. In decision box 609, if a time out occurs before the minimum specified input voltage is reached of inverter 204,404 then inverter is returned to the off or standby state (step 601). Otherwise inverter 204,404 is connected or turned on in step 611. Inverter 204, 404 remains on and connected unless a warning condition (decision box 613) occurs. Possible warning conditions include, (i) disconnection from the electrical grid, (ii) electrical grid stops producing power (islanding), (iii) less than 50 Watts transferred in the last minute, (iv) input voltage to inverter 204,404 is over the maximum limit, and (v) input power is over the maximum limit. If a warning condition occurs (decision box 613) communications signal is turned off (step 615) for inverter 404 or inverter 204 is turned off or put into standby.

The present invention has been described in relation to particular examples, which are intended in all respects to be illustrative rather than restrictive. Those skilled in the art will appreciate that many different combinations of hardware, software, and firmware will be suitable for practicing the present invention. Moreover, other implementations of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. Various aspects and/or components of the described embodiments may be used singly or in any combination in the server arts. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made. 

1. A power string for a distributed power system, comprising: a plurality of DC power sources; a plurality of DC modules, each coupled to a respective one of said DC power sources; an inverter coupled to the DC modules; and a signaling mechanism between said inverter and said DC modules, wherein said signaling mechanism is adapted for controlling operation of said DC modules.
 2. The system according to claim 1, wherein said signaling mechanism superimposes a signal on output of the DC modules.
 3. The system according to claim 1, wherein said signaling mechanism includes a switch integrated with said inverter, said switch modulating a signal onto output of the DC modules.
 4. The system according to claim 1, further comprising: A plurality of receivers, each integrated with respective DC module, said receiver adapted for receiving a signal from said inverter.
 5. The system according to claim 1, further comprising: a detection mechanism in each of said DC modules, adapted for detecting a signal at the frequency of utility electrical grid.
 6. The system according to claim 1, wherein the inverter is coupled to an electric grid and further comprising: a detection mechanism in each of said DC modules, adapted for detecting a signal from said electrical grid and wherein said signal is detected at a higher frequency up converted from the frequency of said electrical grid.
 7. The system according to claim 1, wherein the inverter is coupled to an electric grid and further comprising: a detection mechanism in each of said DC modules, adapted for detecting signals at a switching frequency of said inverter.
 8. The system according to claim 1, wherein each of said DC modules is adapted for operation in a safety mode, and during said safety mode, power output from each DC module is limited.
 9. The system according to claim 8, wherein each of said DC modules includes a detection mechanism, wherein during operation said detection mechanism detects a signal from said inverter and based on said signal said operation of said DC modules is varied from said safety mode of operation to a normal mode of operation for converting power of said DC power sources from said input to said output of said DC modules.
 10. The system of claim 1, wherein output of all of the DC modules is coupled in series.
 11. The system of claim 1, wherein a first subset of the DC modules is coupled in series, a second subset of the DC modules is coupled in series, and the first subset is coupled to the second subset in parallel.
 12. In a distributed power system coupled to electrical grid, the system including: a plurality of DC power source; a power conversion circuit converting power input from said DC power sources and providing converted power; a method comprising selectively operating said power conversion circuit in a safety mode limiting power from said power conversion circuit to thereby avoid electrocution, or in a normal mode for applying the converted power to the grid.
 13. The method according to claim 12, wherein said safety mode is characterized by having less than ten milliamperes of current flow at less than 10 Volts.
 14. The method according to claim 12, wherein the power conversion circuit comprises a plurality of DC modules and an inverter, the method further comprising the steps of: monitoring a signal from said inverter; and upon detecting said signal from said inverter, increasing said power input to said inverter by operating said DC modules in a normal mode.
 15. The method according to claim 14, further comprising a step of: upon detecting said signal and prior to said operating said DC modules in said normal mode of operation, slowly ramping up voltage of said DC modules.
 16. The method according to claim 14, wherein said normal mode of operation of said DC modules includes controlling a maximum peak power at input of said DC modules.
 17. The method according to claim 14, further comprising coupling output of all of the DC modules in series.
 18. The method according to claim 14, further comprising coupling output of a first subset of the DC modules in series, coupling output of a second subset of the DC modules in series, and coupling the first subset to the second subset in parallel.
 19. The method according to claim 14, further comprising during safety mode limiting the current of the DC modules to less than ten milliamperes and the voltage to less than 10 Volts. 